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What's the difference between these two sentence?

He's sure that he'll speak French fluently in a month.

Vs.

He's sure that he'll speak French fluently by a month.

3 Answers 3

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"by a month" is simply not idiomatic at all in English here.

For time, one does things or things are done in a month, week, year, day, hour etc. That is the period of time that will elapse.

He beat the deadline by a month.

There, by a month is used to measure the number of months he won by.

Some project has to be handed in in six months. The guy hands the project in in five months. He has beat the deadline by a month.

For time that will elapse (go by) we use in: in a month.

To measure some amount of time in relation to a set time, we use by:

He beat me by ten minutes. They beat us (in the sailing race) by a month.

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  • @Perplexed folks We use by to specify a time or deadline, as in by tonight, by tomorrow evening, by the end of the week. Dec 12, 2018 at 19:47
  • @RonalsSole So both can be correct in context. Isn't it? Dec 12, 2018 at 19:48
  • @Perplexedfolks Yes, we use in for a duration (a day, a week, a month, a year) and by for a (rough) point in time (5pm, the end of the month etc) Dec 12, 2018 at 20:25
  • @Perplexedfolks You mean to ask: Both can be correct in context, can't they? Answer: No, by a month can never be right in this context in English. By a month: He beat the deadline by a month. [another meaning]
    – Lambie
    Dec 12, 2018 at 20:39
  • @LucianSava No, that is not right. Our contract can be terminated at a month's notice or with a month's notice.
    – Lambie
    Dec 12, 2018 at 20:46
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"In a month" is correct. It means that it will take 1 month by the time he'll speak fluent French. "By" is used to indicate the end point of an event. For example, you can say "He'll speak fluent French by February". February is the deadline by which the activity will be completed.

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  • Thank you for the answer (+^1), but I really don't understand what's the difference between "in" and "by" in these sentence. You explained that "by" indicates end point of an event. For example "He'll be there in a month" = He'll be there after one month from now. It is the same as "by a month". Isn't it? Dec 12, 2018 at 19:53
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    @Perplexedfolks it's not the same, because "a month" is a span of time, and "by" (in this context) takes a point in time. Hence you can say "by February" (because "February" is acting as shorthand for "the start of February", which is a point in time), but not "by a month" (because "a month" isn't a point, but a distance).
    – Darael
    Dec 12, 2018 at 20:41
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    @Perplexedfolks In terms of a specific point in time and the use of by, you would say by next month. (Note the lack of a.) Further, it would not be natural to say in next month. Dec 12, 2018 at 20:47
  • @Perplexedfolks I will finish in ten minutes. Same thing. When you are referring to an amount of time, use in: to start in ten minutes, to start in two years. For deadlines, by six o'clock, by 2021.
    – Lambie
    Dec 12, 2018 at 20:50
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The primary difference is that one of those two sentences is grammatical, and the other is not.

"He's sure that he'll speak French fluently in a month"

is correct. It is equivalent to "He's sure that he'll speak French fluently within a month", or "He's sure that before a month has passed, he'll speak French fluently". In this context, "[with]in [some amount of time]" means "before [that much time] has elapsed".

"He's sure that he'll speak French fluently by a month"

is not grammatical, because in this context "by [something]" means "before [something] has happened". "In" needs a span - a length of time, which might begin now, or at some other point already established by context ("once he starts the course, he's sure that he'll speak French fluently in a month" would mean the month started when the course did, at some point in the future, rather than right now).

"By" needs an point in time: "a month" isn't a point, but "next month" could be (in this context it would be taken to mean "the start of next month"). Hence you might say "he's sure that he'll speak French fluently by summer", because "[the start of] summer" is a point. So is "dinner", though expecting someone to gain fluency between now and dinner is probably unreasonable. You can use "by the time [something happens]" in a similar way, so you could say "by the time a month is up", where "the time a month is up" means the point in time at which one complete month has passed.

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  • Or you could say "I'm sure she'll be fluent by dinner time" or "I'm sure he'll be fluent by Summer." (which is probably a little more realistic ;) )
    – ColleenV
    Dec 12, 2018 at 21:05
  • @ColleenV Thanks; I was blanking on good events to use as examples but those are much better and I shall edit accordingly.
    – Darael
    Dec 12, 2018 at 21:06
  • "By" does not "need an event". "By" needs a deadline for doing something i.e. a date, a time, specified or implied. He will be home by dinnertime. "Can you finish this job by 3:00 o'clock?"
    – Lambie
    Dec 12, 2018 at 23:53
  • @Lambie you are correct; "event" is not quite the right word, but I was focusing on distinguishing points in time from spans of time, and when I wrote the answer I felt the phrasing I've used in this comment might be confusing. I don't remember why I thought that, only that I did.
    – Darael
    Dec 13, 2018 at 1:30
  • @Darael why not to use "a point of time" instead of an "event"? Dec 13, 2018 at 10:31

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